The enhancer element of human papovavirus BKV has been characterized by testing BKV enhancer subclones in a transient expression assay. Like its monkey counterpart, simian virus 40 (SV40), the BKV enhancer activates transcription of heterologous genes in human, monkey or mouse cells. Although the BKV and SV40 enhancer sequences are different, certain shared oligonucleotide motifs may be responsible for the similar function of the two enhancer elements. Studies are in progress to characterize further the contribution of these homologous sequences to enhancer activity.